The Dart VM is single-threaded, so all of an app's code runs in one thread, also called the main isolate. This is because main() is the function where Dart code starts executing an isolate, because Dart's concurrency model is based on isolates as separate processes that exchange messages. We will talk about isolates in depth in the coming recipes, but if your code doesn't start a new isolate, all of it runs in one isolate. But, in this one isolate, you can have lots of asynchronous pieces of code (let's call them tasks) running at the same time; in what order do they execute, and can we influence that order? It turns out that a better understanding of Dart's event loop and task queuing mechanism enables us to do that. This recipe will clarify Dart's scheduling mechanism and give you hints and tips for an ordered execution of tasks.
Have a look at the program tasks_scheduling.dart (the tasks are numbered consecutively and according to the way they are started):
import'dart:async';
main() {
print('1) main task #1'),
scheduleMicrotask(() => print('2) microtask #1'));
newFuture.delayed(new Duration(seconds:1),
() =>print('3) future #1 (delayed)'));
new Future(() => print('4) future #2'));
print('5) main task #2'),
scheduleMicrotask(() => print('6) microtask #2'));
new Future(() => print('7) future #3'))
.then((_) => print('8) future #4'))
.then((_) => print('9) future #5'))
.whenComplete(cleanup);
scheduleMicrotask(() => print('11) microtask #3'));
print('12) main task #3'),
}
cleanup() {
print('10) whenComplete #6'),
}The following screenshot shows the output of the program, the order of which is explained in the next section:
The main isolate proceeds as follows:
- First the code in
main()is executed, line after line and synchronously. - Then the
event-loopmechanism kicks in, and this looks at the two queues in the system in the following order:- First, it takes a look at the microtask queue this queue is for all tasks that need to be executed before the event queue, for example, the changes that have to be done internally before the DOM starts rendering the modified screen. All of the tasks in this queue are executed before going to the following step.
- Then, the event queue is handled, here, the tasks of all outside events, such as mouse events, drawing events, I/O, timers, messages between Dart isolates, and so on, are scheduled. Of course, in each queue, the tasks are handled one by one in the first in first out order.
In principle, when both queues are empty, the app can exit. Tasks (these are pieces of code to run later, asynchronously) can be scheduled using the following classes and methods from dart:async:
- Make a new Future object with a function to execute; this is appended to the event queue.
- With
Future.delayed, you can specify the execution of a function to occur after a certain duration; this also goes to the event queue. - Call the top-level
scheduleMicrotask()function, which appends an item to the microtask queue.
Chaining Futures in a series of then statements effectively ensures that they are executed in that order (see step 1 of the previous recipe). Also, a whencomplete clause will execute immediately after all the previous then statements.
So this is the order of execution: main(), then à microtask queue, then event queue, and then delayed tasks.